This project is concerned with cellular regulatory factors affecting DNA replication, particularly those pertaining to the enzymology and control of DNA precursor synthesis. Several lines of evidence indicate that the enzymes of deoxyribonucleotide biosynthesis interact to form a specific complex, which is juxtaposed with the DNA replication apparatus. The action of this complex can compartmentalize DNA precursors and maintain high concentrations at the replication fork. Using the well-characterized bacteriophage T4, we hope to further explore this model. We shall attempt to purify the complex from T4-infected Escherichia coli and to determine protein-protein relationships among its constituent enzymes by use of cross-linking reagents, fluorescence measurements, and attempted reconstitution. We shall seek evidence that the complex is associated with elements of the molecular properties of the enzymes involved and regulation of the genes which encode them. Finally, we shall study aspects of DNA precursor metabolism in T7 phage infection - aspects which should deepen our knowledge of how the enzymes of deoxyribonucleotide biosynthesis are organized intracellularly. The project remains essentially unchanged, except for two additional sub-projects: We describe methods for estimation of the effective concentrations of deoxyribonucleoside triphosphates at replication forks in intact cells. Also the complex we are studying is controlled in an unusual manner which we hope to elucidate.